Humidity or moisture swing sorbents are materials with affinity to carbon dioxide (CO2) that can be modified substantially by the presence or absence of water. Conventional moisture swing sorbents bind CO2 when relatively dry, and release them again when exposed to increased levels of moisture, either in the form of liquid water or water vapor. Some moisture swing sorbents can comprise polymers with quaternary ammonium ions attached to the polymer matrix and anions that are mobile in the polymer matrix. The material is active if large fractions of the anions are hydroxide, carbonate or bicarbonate ions. A typical conventional moisture swing sorbent is a strong-base anionic exchange resin. For specific resins studied in the past, the equilibrium partial pressure of CO2 over the resin at room temperature can increase about five hundred fold as the humidity moves from 5 parts per thousand to fully saturated air (around 30 parts per thousand) or the resin is brought in contact with liquid water. In such systems, it is possible to capture carbon dioxide from ambient air (about 400 ppmV of CO2 in the air) and release it at a partial pressure in the 1 to 10% range (1%=10,000 ppmV). It is possible to amplify the pressure boost by raising the temperature of the sorbent during regeneration. The moisture swing has been observed to release CO2 from the sorbent in the bicarbonate until the material returns to what is effectively the “carbonate” state. The maximum possible size of the moisture swing is set by the concentration of positive ionic sites in the sorbent. For materials studied, the charge density is between two and three moles per kilogram. In the bicarbonate state the sorbent holds one CO2 molecule for every positive charge, in the carbonate state it holds one CO2 for every two positive charges. This is the practical size of the swing. This suggests that the CO2 being released in a swing that is of the order of a few percent of the weight of the sorbent.
All well studied sorbents have in common that they are relatively brittle, glass-like materials that nevertheless swell significantly when exposed to moisture. The strains associated with swelling have so far made it impossible to create large structures from homogeneous sorbent material. This has prevented, for example, extruding simple monoliths or long thin strands of pure sorbent materials. On the other hand, fine powders can easily tolerate the strains and stresses associated with wetting and drying the sorbent. This in turn has resulted in composite materials in which sorbent powders are embedded into a matrix. For example, the Snowpure membranes (SnowPure, LLC, 130-A Calle Iglesia, San Clemente, Calif. 92672, USA) include resin particles that form about two thirds of the weight of the overall membrane. By packaging sorbent powders behind protective barriers, at least some air flow can permeate the barrier while protecting the sorbent from direct contact with salt water without stopping the exposure to moisture. Moreover, the effective use of a physical barrier can prevent chloride or sulfate ions (that may for example be present in salt water) to directly contact sorbent materials. The direct contact of salty water with a sorbent such as an ion exchange material can deactivate the material. During exposure to salty water, carbonate ions can be displaced with anions present in the salt, and the resin can cease to be a carbon dioxide sorbent. It is therefore a great advantage to have introduced a barrier to liquid water that prevents the exchange of ions. This type of barrier technology can make it possible to reduce the dependence on fresh water that until now was inherent in the humidity swing design.